Invisible Web

The so-called invisible Web is data that's locked up in databases, or it's in non-text formats, but in either case, it's
hidden from search engines. Also, What is the Invisible Web? (UC Berkeley libraries)

Multiple Search Engines

Some of these sites (also known as metasearch engines) let you search several search engines all at once; others list many search engines on a single page (these are sometimes called all-in-one pages).